In assembling a string of tubing for an oil well, a long series of pipe sections are screwed together, usually by means of a power tongs. Conventional power tongs have jaws with serrated faces that are designed to bite into the pipe surface. These jaws are usually cam operated, so that their closing force is roughly proportional to the tongs torque. Such an arrangement, however, is unsuitable for certain modern tube materials which are particularly soft. This is true for example, for some high chromium pipes designed for corrosion resistance rather than strength, and for fiber glass tubing now being developed. There are also pipes having protective coatings designed to protect the underlying material from corrosion by hydrogen sulfide and other subterranean chemicals. Some tubing costs on the order of one thousand dollars per foot, and a string is typically thousands of feet long. Plainly, it is critical not to damage tubing while making up a string, and to avoid any surface damage that could reduce service life.
Consequently, there is a need for power tongs whose jaws will not injure the surface of tubing and yet can deliver to the tubing adequate torque, which may be several thousands foot-pounds. The approach taken by this invention is to provide jaws actuated by hydraulic linear motors so that jaw pressure can be accurately controlled.
Hydraulically biased jaws on power tongs are known in the prior art; see, for example, U.S. Pat. Nos. 3,796,418, 3,921,473, 4,057,887 and No. 4,402,293. None of these however, provide means for independently controlling the jaw force.
It is therefore an object of this invention to construct a power tongs having jaws designed not to bite into the surface of pipe or tubing and yet able to transfer sufficient torque to the tubing.
Another object is to control jaw pressure independent of tongs torque. These and other objects will be apparent from the following description of a preferred embodiment of the invention.
The foregoing objects are met, according to this invention, by a power tongs having at least one hydraulically biased jaw, and means for controlling the hydraulic pressure delivered to the jaw. With this arrangement, smooth-faced jaws made of soft material can be used, and purely frictional engagement with the pipe surface suffices to make up the string.